Determining one or more profile parameters of a photomask covered by a pellicle

ABSTRACT

Provided is a method of determining one or more profile parameters of a photomask covered with a pellicle, the method comprising developing an optical metrology model of a pellicle covering a photomask, developing an optical metrology model of the photomask, the photomask separated from the pellicle by a medium and having a structure, the structure having profile parameters, the optical metrology model of the photomask taking into account the optical effects on the illumination beam transmitted through the pellicle and diffracted by the photomask structure. The optical metrology model of the pellicle and the optical metrology model of the photomask structure are integrated and optimized. At least one profile parameters of the photomask structure is determined using the optimized integrated optical metrology model.

BACKGROUND

1. Field

The present application generally relates to optical metrology of astructure formed on a semiconductor wafer, and, more particularly, todetermining one or more profile parameters of a patterned photomaskcovered by a pellicle using optical metrology.

2. Related Art

In semiconductor manufacturing, periodic gratings are typically used forquality assurance. For example, one typical use of periodic gratingsincludes fabricating a periodic grating in proximity to the operatingstructure of a semiconductor chip. The periodic grating is thenilluminated with an electromagnetic radiation. The electromagneticradiation that deflects off of the periodic grating are collected as adiffraction signal. The diffraction signal is then analyzed to determinewhether the periodic grating, and by extension whether the operatingstructure of the semiconductor chip, has been fabricated according tospecifications.

In one conventional system, the diffraction signal collected fromilluminating the periodic grating (the measured diffraction signal) iscompared to a library of simulated diffraction signals. Each simulateddiffraction signal in the library is associated with a hypotheticalprofile. When a match is made between the measured diffraction signaland one of the simulated diffraction signals in the library, thehypothetical profile associated with the simulated diffraction signal ispresumed to represent the actual profile of the periodic grating.

The hypothetical profiles, which are used to generate the simulateddiffraction signals, are generated based on a profile model thatcharacterizes the structure to be examined. Thus, in order to accuratelydetermine the profile of the structure using optical metrology, aprofile model that accurately characterizes the structure should beused.

SUMMARY

Provided is a method of determining one or more profile parameters of aphotomask covered with a pellicle, the method comprising developing anoptical metrology model of a pellicle covering a photomask, developingan optical metrology model of the photomask, the photomask separatedfrom the pellicle by a medium and having a structure, the structurehaving profile parameters, the optical metrology model of the photomasktaking into account the optical effects on the illumination beamtransmitted through the pellicle and diffracted by the photomaskstructure. The optical metrology model of the pellicle and the opticalmetrology model of the photomask structure are integrated and optimized.At least one profile parameters of the photomask structure is determinedusing the optimized integrated optical metrology model.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is an architectural diagram illustrating an exemplary embodimentwhere optical metrology can be utilized to determine the profiles ofstructures formed on a semiconductor wafer.

FIG. 1B depicts an exemplary one-dimension repeating structure.

FIG. 1C depicts an exemplary two-dimension repeating structure

FIG. 2A depicts exemplary orthogonal grid of unit cells of atwo-dimension repeating structure.

FIG. 2B depicts a top-view of a two-dimension repeating structure.

FIG. 2C is an exemplary technique for characterizing the top-view of atwo-dimension repeating structure.

FIG. 3 is an exemplary architectural diagram of the optical model of aphotomask covered with a pellicle.

FIG. 4A is an exemplary architectural diagram of the optical model ofthe pellicle whereas FIG. 4B is an exemplary architectural diagram of aphotomask covered by more than one layer of the pellicle.

FIG. 5 is an exemplary flowchart for developing an optimized metrologymodel of photomask covered by a pellicle and for determining profileparameters of the photomask structure.

FIG. 6 is an exemplary block diagram of a system for utilizing a librarydeveloped for determining the profile parameters of photomask structurecovered by a pellicle.

FIG. 7 is an exemplary block diagram of a system for utilizing a machinelearning system developed for determining the profile parameters ofphotomask structure covered by a pellicle.

FIG. 8 is an exemplary flowchart for determining and utilizing metrologydata for automated process and equipment control.

FIG. 9 is an exemplary block diagram for determining and utilizingmetrology data for automated process and equipment control.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT(S)

In order to facilitate the description of the present invention, asemiconductor wafer may be utilized to illustrate an application of theconcept. The methods and processes equally apply to other work piecesthat have repeating structures. Furthermore, in this application, theterm structure when it is not qualified refers to a patterned structure.

FIG. 1A is an architectural diagram illustrating an exemplary embodimentwhere optical metrology can be utilized to determine the profiles orshapes of structures fabricated on a semiconductor wafer. The opticalmetrology system 40 includes a metrology beam source 41 projecting ametrology beam 43 at the target structure 59 of a wafer 47. Themetrology beam 43 is projected at an incidence angle θ towards thetarget structure 59. The diffracted beam 49 is measured by a metrologybeam receiver 51. A measured diffraction signal 57 is transmitted to aprofile server 53. The profile server 53 compares the measureddiffraction signal 57 against a library 60 of simulated diffractionsignals and associated hypothetical profiles representing varyingcombinations of critical dimensions of the target structure andresolution. In one exemplary embodiment, the library 60 instance bestmatching the measured diffraction signal 57 is selected. Thehypothetical profile and associated critical dimensions of the selectedlibrary 60 instance are assumed to correspond to the actualcross-sectional shape and critical dimensions of the features of thetarget structure 59. The optical metrology system 40 may utilize areflectometer, an ellipsometer, or other optical metrology device tomeasure the diffraction beam or signal. An optical metrology system isdescribed in U.S. Pat. No. 6,913,900, entitled GENERATION OF A LIBRARYOF PERIODIC GRATING DIFFRACTION SIGNAL, issued on Sep. 13, 2005, whichis incorporated herein by reference in its entirety.

Simulated diffraction signals can be generated by applying Maxwell'sequations and using a numerical analysis technique to solve Maxwell'sequations. It should be noted that various numerical analysistechniques, including variations of rigorous coupled wave analysis(RCWA), can be used. For a more detail description of RCWA, see U.S.Pat. No. 6,891,626, titled CACHING OF INTRA-LAYER CALCULATIONS FOR RAPIDRIGOROUS COUPLED-WAVE ANALYSES, filed on Jan. 25, 2001, issued May 10,2005, which is incorporated herein by reference in its entirety.

Simulated diffraction signals can also be generated using a machinelearning system (MLS). Prior to generating the simulated diffractionsignals, the MLS is trained using known input and output data. In oneexemplary embodiment, simulated diffraction signals can be generatedusing an MLS employing a machine learning algorithm, such asback-propagation, radial basis function, support vector, kernelregression, and the like. For a more detailed description of machinelearning systems and algorithms, see U.S. patent application Ser. No.10/608,300, titled OPTICAL METROLOGY OF STRUCTURES FORMED ONSEMICONDUCTOR WAFERS USING MACHINE LEARNING SYSTEMS, filed on Jun. 27,2003, which is incorporated herein by reference in its entirety.

The term “one-dimension structure” is used herein to refer to astructure having a profile that varies in one dimension. For example,FIG. 1B depicts a periodic grating having a profile that varies in onedimension (i.e., the x-direction). The profile of the periodic gratingdepicted in FIG. 1B varies in the z-direction as a function of thex-direction. However, the profile of the periodic grating depicted inFIG. 1B is assumed to be substantially uniform or continuous in they-direction.

The term “two-dimension structure” is used herein to refer to astructure having a profile that varies in two-dimensions. For example,FIG. 1C depicts a periodic grating having a profile that varies in twodimensions (i.e., the x-direction and the y-direction). The profile ofthe periodic grating depicted in FIG. 1C varies in the z-direction.

Discussion for FIGS. 2A, 2B, and 2C below describe the characterizationof two-dimension repeating structures for optical metrology modeling.FIG. 2A depicts a top-view of exemplary orthogonal grid of unit cells ofa two-dimension repeating structure. A hypothetical grid of lines issuperimposed on the top-view of the repeating structure where the linesof the grid are drawn along the direction of periodicity. Thehypothetical grid of lines forms areas referred to as unit cells. Theunit cells may be arranged in an orthogonal or non-orthogonalconfiguration. Two-dimension repeating structures may comprise featuressuch as repeating posts, contact holes, vias, islands, or combinationsof two or more shapes within a unit cell. Furthermore, the features mayhave a variety of shapes and may be concave or convex features or acombination of concave and convex features. Referring to FIG. 2A, therepeating structure 300 comprises unit cells with holes arranged in anorthogonal manner. Unit cell 302 includes all the features andcomponents inside the unit cell 302, primarily comprising a hole 304substantially in the center of the unit cell 302.

FIG. 2B depicts a top-view of a two-dimension repeating structure. Unitcell 310 includes a concave elliptical hole. In FIG. 2B, unit cell 310includes a feature 320 that comprises an elliptical hole, where thedimensions become progressively smaller until the bottom of the hole.Profile parameters used to characterize the structure includes theX-pitch 310 and the Y-pitch 314. In addition, the major axis of theellipse 316 that represents the top of the feature 320 and the majoraxis of the ellipse 318 that represents the bottom of the feature 320may be used to characterize the feature 320. Furthermore, anyintermediate major axis between the top and bottom of the feature mayalso be used as well as any minor axis of the top, intermediate, orbottom ellipse, (not shown).

FIG. 2C is an exemplary technique for characterizing the top-view of atwo-dimension repeating structure. Unit cell 330 includes a feature 332,an island with a peanut-shape viewed from the top. One modeling approachincludes approximating the feature 332 with a variable number orcombinations of ellipses and polygons. Assume further that afteranalyzing the variability of the top-view shape of the feature 332, itwas determined that two ellipses, Ellipsoid 1 and Ellipsoid 2, and twopolygons, Polygon 1 and Polygon 2, were found to fully characterizefeature 332. In turn, parameters needed to characterize the two ellipsesand two polygons comprise nine parameters as follows: T1 and T2 forEllipsoid 1; T3, T4, and θ₁ for Polygon 1; T4, T5, and θ₂ for Polygon 2;and T6 and T7 for Ellipsoid 2. Many other combinations of shapes couldbe used to characterize the top-view of the feature 332 in unit cell330. For a detailed description of modeling two-dimension repeatingstructures, refer to U.S. patent application Ser. No. 11/061,303,OPTICAL METROLOGY OPTIMIZATION FOR REPETITIVE STRUCTURES, filed on Apr.27, 2004, which is incorporated herein by reference in its entirety.

FIG. 3 is an exemplary architectural diagram of the optical model of aphotomask covered with a pellicle. The illumination beam 402 is directedthrough the pellicle 420 at an angle, which can be zero from normal. Theillumination beam 402 strikes the upper surface of the pellicle 420where part of the beam is reflected off the upper surface as beam 412.Part of the illumination beam passes through the pellicle 420 as beam403 and strikes the lower surface of the pellicle 420. Part of the beam403 going through the pellicle is reflected as beam 411 through thepellicle 420 and moves away from the surface of the pellicle 420. Partof the illumination beam 404 that is not reflected by the upper andlower surfaces of the pellicle goes through the medium 440 and strikesthe photomask structure 406. The illumination beam that goes through thepellicle, beam 404, is determined by the thickness t of the pellicle 420and the refractive indices, (refractive index, N, and extinctioncoefficient, K), of the pellicle, and the angle of incidence of theillumination 402.

Diffraction of the incident beam 404 is affected by the photomaskstructure and by the refractive indices of the materials used in thephotomask structure 406. The signal off the photomask structure 406 isthe detection beam 408. The detection beam 408 strikes the lower surfaceof the pellicle 420 and part of the detection beam 408 is reflected backto the photomask structure as beam 415. Part of the detection beam 408goes through the pellicle material 420 as the beam 409 strikes the uppersurface of the pellicle 420. Part of the detection beam 409 is reflectedback to the photomask structure 420 as beam 410. Part of the detectionbeam 409 passing through the pellicle 420 proceeds to the detector (notshown) as beam 414. Beam 409 is determined by the thickness t of thepellicle 420, the refractive indices, (N and K), of the pellicle, andthe angle of incidence of the detection beam 408.

The pellicle 420 may be a polymer film or a glass plate. The pelliclematerial is required to be transparent to the lithography wavelength.The medium 440 between the pellicle 420 and the photomask structure 406is typically air but maybe a liquid. The photomask structure istypically fabricated on a quartz 430 plate. The detector may be aspectrometer. The photomask structure covered with a pellicle 400 may bemeasured with an optical metrology device such as a broadbandreflectometer, a broadband ellipsometer, and the like.

FIG. 4A is an exemplary architectural diagram of the optical model 500of the pellicle 540 depicting more detail of the propagation process ofthe illumination beam 502. Illumination beam 502 at an angle ofincidence θ hits the upper surface of pellicle 540 where part of theillumination beam is reflected as beam 504. As mentioned above, θ can bezero or greater than zero. As mentioned above, the incident beam 506that goes through the pellicle is affected by the thickness t of thepellicle 540 and the N and K of the pellicle 540. The beam 506 goingthrough the pellicle 540 is partially reflected back through thepellicle 540 when the beam 506 hits the lower surface of the pellicle540. Similarly, the beam 508 that hits the lower surface of the pellicle540 is partially reflected back as beam 514, the beam 514 gettingpartially reflected back as beam 518 to the upper surface of thepellicle 540. The process is repeated several times where the strengthof the beam bouncing back and forth inside the pellicle diminishes aftereach iteration. FIG. 4B is an exemplary architectural diagram of aphotomask covered by more than one layer of the pellicle. The layers ofthe pellicle may comprise two or more layers of polymer film or two ormore glass plates. The photomask structure 600 is covered with twopellicle layers 602 and 604. As is well known in the art, the pelliclelayers 602 and 604 are separated from the photomask structure 610 by themedium 606. As will be shown below, the optical modeling and equationfor two or more layers of the pellicle are handled in a similar manneras calculations related to the propagation of light through thin filmlayers of wafer structures. For details of the calculations, refer toU.S. Pat. No. 6,891,626 CACHING OF INTRALAYER CALCULATIONS FOR RAPIDRIGOROUS COUPLE-WAVE ANALYSIS, by Niu, issued on May 10, 2005,incorporated herein by reference in its entirety.

The equations for modeling the coherent and incoherent portions of thebeam diffraction are discussed separately. Coherent beam calculationsapply to the reflections of the illumination or detection beams beingreflected at the beam hits the upper or lower surface of the pellicle.The basic reflectivity equation for a polarized beam is as follows:

$\begin{matrix}{{R_{s,p} = \frac{r^{s,p} + {r_{1}^{s,p} \cdot ^{\varphi}}}{1 + {r^{s,p} \cdot r_{1}^{s,p} \cdot ^{\varphi}}}}{where}{\varphi = {\frac{4\pi \; t}{\lambda}\sqrt{ɛ - {{Sin}^{2}\theta}}}}} & (1.00)\end{matrix}$

t is the thickness of the pellicle,

where

i=√{square root over (−1)} and

ε=(n+ik)²

n and k are the refraction index and coefficient of extinction of thepellicle,

θ is the AOI

r₁ is the diffracted beam across the thickness of the pellicle

r_(s) is the reflectivity on the s component of a polarized beam

r_(p) is the reflectivity on the p component of a polarized beam.

Specific equations to get r_(s), r_(p) etc. are:

$r_{s} = \frac{z_{1} - z_{2}}{z_{1} + z_{2}}$$r_{p} = \frac{{z_{1}/ɛ_{1}} - {z_{2}/ɛ_{2}}}{{z_{1}/ɛ_{1}} + {z_{2}/ɛ_{2}}}$where: $z_{1} = \sqrt{ɛ_{1} - {\sin^{2}\theta}}$$z_{2} = {\sqrt{ɛ_{2} - {\sin^{2}\theta}}.}$

The reflection of the incident beam that hits the upper pellicle surfacemay or may not affect the spectrometer measurement depending on depth offocus of the beam and the angle of incidence (AOI). If a normal AOI beamis used, then the reflections are part of the measured diffractionsignal off the photomask structure covered by the pellicle. If the beamhas a non-normal AOI, then if the depth of focus is large, then thereflections are part of the measured diffraction signal off thephotomask structure covered by the pellicle. Otherwise, the reflectionsare shifted so that these will not be part of the measurement.

The illumination and detection beams that travel through the medium 440are modeled using the regular optical modeling methods described in thereferences above. Modeling for the reduction of beam intensity whilepassing through material layers and the medium is depicted in FIG. 3 bynoting how the beam intensity changes. Referring to FIG. 3, theillumination beam 402 has a transmission E, after reflection on hittingthe upper surface, the beam 402 has a transmission of {tilde over (T)},and the beam moving through the medium 440, the transmission is {tildeover (T)}•E. The detection beam 408 has a transmission of D•{tilde over(T)}•E where D is the diffraction of the detection beam 408. Thedetection beam becomes the beam 409 with a transmission through thepellicle away from the photomask is T, and the detection beam 414towards the spectrometer (not shown) has a transmission ofT_(AVG)=T•D•{tilde over (T)}•E. The detection beam measured by thespectrometer as an average over the spectrometer resolution is:

Measurement=√{square root over (<|{tilde over (T)}∥T²|>•D)}  (1.10)

where D is the diffraction of the detection beam from the photomaskstructures,

T is the transmission through the pellicle when the light moves towardthe photomask, and

E is transmission through the pellicle when the light moves away fromthe photomask.

FIG. 5 is an exemplary flowchart for developing an optimized metrologymodel of photomask covered by a pellicle and for determining profileparameters of the photomask structure. In step 702, an optical metrologymodel of the pellicle is developed. Included in the model is thethickness of the pellicle, the N&K refractive indices, and the angle ofincidence of the illumination beam or the detection beam. In step 704,the optical metrology model of the photomask structure is developed. Themodeling process is similar to modeling of regular wafer patternedlayers. In step 706, the optical models of the pellicle and thephotomask structure model are integrated. Integration includes ensuringthe model is seamlessly combined to facilitate simulation of the signaldiffracted off the photomask structure covered with a pellicle. In step708, the integrated optical model is optimized. For details onoptimizing a model, refer to U.S. patent application Ser. No.10/206,491, titled MODEL AND PARAMETER SELECTION FOR OPTICAL METROLOGY,filed on Jul. 25, 2002, which is incorporated herein by reference in itsentirety. In step 710, measured diffraction signals and the optimizedintegrated optical model are used to determine at least one parameter ofthe photomask structure. The determined parameter may be compared to anacceptable range of values and flagged if outside the range.Determination of the parameters may be done using regression, a library,or a machine learning system. The optimized model may be used to developa library of pairs of simulated diffraction signals and parameters ofthe photomask structure covered by the pellicle. Alternatively, a set ofsimulated diffraction signals and parameters of the photomask structurecovered by the pellicle may be used to train a machine learning systemto accept diffraction signals as input and generate parameters of thephotomask structure covered with a pellicle as output.

FIG. 6 is an exemplary block diagram of a system for utilizing a librarydeveloped for determining the profile parameters of photomask structurecovered by a pellicle. In one exemplary embodiment, optical metrologysystem 804 can also include a library 810 with a plurality of simulateddiffraction signals and a plurality of values of one or more photomaskparameters associated with the plurality of simulated diffractionsignals. As described above, the library can be generated in advance,metrology processor 808 can compare a measured diffraction signal off astructure to the plurality of simulated diffraction signals in thelibrary When a matching simulated diffraction signal is found, the oneor more values of the one or more photomask parameters associated withthe matching simulated diffraction signal in the library is assumed tobe the one or more values of the one or more photomask parameters.

FIG. 7 is an exemplary block diagram of a system for utilizing a machinelearning system developed for determining the profile parameters ofphotomask structure covered by a pellicle. System 1100 includes aphotolithography cluster 1102 and an optical metrology system 1104.Photolithography cluster 1102 is configured to perform a waferapplication to fabricate a structure on a wafer. Optical metrologysystem 1104 includes a beam source and detector 1106, processor 1108,and machine learning system 1110. Beam source and detector 1106 can becomponents of a scatterometry device, such as a reflectometer,ellipsometer, and the like. Beam source and detector 1106 are configuredto measure a set of diffraction signals off the photomask structurecovered by a pellicle. Processor 1108 is configured to train machinelearning system 1110 using the set of measured diffraction signals asinputs to machine learning system 1110 and the set of different valuesfor the one or more photomask parameters as the expected outputs ofmachine learning system 1110.

After machine learning system 1110 has been trained, optical metrologysystem 1100 can be used to determine one or more values of one or morephotomask parameters of a wafer application. In particular, a structureis fabricated using photolithography cluster 1102 or anotherphotolithography cluster. A diffraction signal is measured off thestructure using beam source and detector 1106. The measured diffractionsignal is inputted into the trained machine learning system 1110 toobtain one or more values of one or more photomask parameters as anoutput of the trained machine learning system 1110.

FIG. 8 is an exemplary flowchart for determining and utilizing metrologydata for automated process and equipment control. In step 1302, anintegrated optical metrology model of a photomask covered with pellicleis optimized. In step 1304, measurements off the photomask covered bythe pellicle are obtained using an optical metrology device such as areflectometer or an ellipsometer. In step 1306, the one or more profileparameters of the photomask structure is determined using the optimizeddiffraction signal and the measured diffraction signal. As mentionedabove, determination of the one or more profile parameters may be doneusing the optimized optical metrology model and regression, or by usingthe optimized optical metrology model to create a library of pairs ofsimulated diffraction signals and set of profile parameters, or by thecreating a trained machine learning system with measured diffractionsignals as input and the one or more profile parameters of the photomaskstructure as output.

In step 1308, at least one determined profile parameter value of thephotomask structure is transmitted to the lithography cluster. Thelithography cluster may include fabrication equipment that utilizes thephotomask structure for exposure, development, and thermal processes ofa substrate with a resist layer. In step 1312, at least one processparameter or equipment setting of the lithography cluster based on thetransmitted at least one determined profile parameter of the photomaskstructure. For example, a top CD or a bottom CD of the photomaskstructure may be transmitted to the photolithography cluster. The top CDor bottom CD value may be used as the basis for modifying the focus ordose of the exposure equipment or the temperature or length of baking inthe post exposure baking process. After at least one process step in thelithography cluster is completed, in step 1314, the fabricated structureis measured using an optical metrology device, such as a reflectometeror an ellipsometer, or a CDSEM. In step 1314, at least one profileparameter of the fabricated structure is determined based on themeasurement with the optical metrology device. In step 1316, acorrelation is developed between the transmitted profile parameter ofthe photomask structure and one or more parameters determined after thelithography processing. For example, assume that a top CD of thephotomask structure was transmitted to the lithography cluster. After anexposure and development process is performed on the resist in thelithography cluster, the resist structure profile is determined in step1314. One or more determined profile parameter of the resist structureis correlated with the top CD of the photomask structure. Thiscorrelation may be used to adaptively change which process parameter orequipment setting is modified and by how much, in step 1312.

FIG. 9 depicts an exemplary system 1400 to control a photolithographycluster. System 1400 includes a photolithography cluster 1402 andoptical metrology system 1404. System 1400 also includes a fabricationcluster 1406. Although fabrication cluster 1406 is depicted in FIG. 14as being subsequent to photolithography cluster 1402, it should berecognized that fabrication cluster 1406 can be located prior tophotolithography cluster 1402 in system 1400.

A photolithographic process, such as exposing and/or developing aphotomask layer applied to a wafer, can be performed usingphotolithography cluster 1402. Optical metrology system 1404 is similarto optical metrology system 40 (FIG. 1A). In one exemplary embodiment,optical metrology system 1404 includes a beam source and detector 1408and processor 1410. Beam source and detector 1408 are configured tomeasure a diffraction signal off the structure. Processor 1410 isconfigured to compare the measured diffraction signal to a simulateddiffraction signal. The simulated diffraction signal was generated usingone or more values of one or more profile parameters of the photomaskstructure. The one or more values of the one or more profile parametersused to generate the simulated diffraction signal were obtained from theone or more values of the one or more photomask parameters associatedwith the simulated diffraction signal. If the measured diffractionsignal and the stored simulated diffraction signal match, one or morevalues of the photomask parameters in the fabrication application aredetermined to be the one or more values of the photomask parametersassociated with the stored simulated diffraction signal.

In one exemplary embodiment, optical metrology system 1404 can alsoinclude a library 1412 with a plurality of simulated diffraction signalsand a plurality of values of one or more photomask parameters associatedwith the plurality of simulated diffraction signals. As described above,the library can be generated in advance, metrology processor 1410 cancompare a measured diffraction signal off a structure to the pluralityof simulated diffraction signals in the library When a matchingsimulated diffraction signal is found, the one or more values of the oneor more photomask parameters associated with the matching simulateddiffraction signal in the library is assumed to be the one or morevalues of the photomask parameters used in the wafer application tofabricate the structure.

System 1400 also includes a metrology processor 1414. In one exemplaryembodiment, processor 1410 can transmit the one or more values of theone or more photomask parameters to metrology processor 1414. Metrologyprocessor 1414 can then adjust one or more process parameters orequipment settings of photolithography cluster 1402 based on the one ormore values of the one or more photomask parameters determined usingoptical metrology system 1404. Metrology processor 1414 can also adjustone or more process parameters or equipment settings of fabricationcluster 1406 based on the one or more values of the one or morephotomask parameters determined using optical metrology system 1404. Asnoted above, fabrication cluster 1406 can process the wafer before orafter photolithography cluster 1402

Although exemplary embodiments have been described, variousmodifications can be made without departing from the spirit and/or scopeof the present invention. Therefore, the present invention should not beconstrued as being limited to the specific forms shown in the drawingsand described above.

1. A method of determining one or more profile parameters of a photomaskcovered with a pellicle, the method comprising: developing an opticalmetrology model of a pellicle covering a photomask, the pellicle havinga first and second surface; developing an optical metrology model of thephotomask, the photomask separated from the pellicle by a medium andhaving a structure, the structure having profile parameters, the opticalmetrology model of the photomask taking into account the optical effectson the illumination beam transmitted through the pellicle and diffractedby the photomask structure, the diffraction generating a detection beam;integrating the optical metrology model of the pellicle and the opticalmetrology model of the photomask, generating an integrated opticalmetrology model; optimizing the integrated optical metrology model; anddetermining one or more profile parameters of the mask structure usingthe optimized integrated optical metrology model.
 2. The method of claim1 wherein the optical metrology model of the pellicle comprises: (a)developing a first optical metrology model of the pellicle, the firstoptical metrology model of the pellicle taking into account opticaleffects on the illumination beam related to the first and secondsurfaces of the pellicle; and (b) developing a second optical metrologymodel of the pellicle, the second optical metrology model of thepellicle taking into account optical effects on the detection beamrelated to the first and second surfaces of the pellicle.
 3. The methodof claim 2 wherein the first optical metrology model of the pelliclecovering the photomask takes into account the optical effects on theillumination beam related to reflection off the first surface of thepellicle, refractive indices of the pellicle, and reflection of theillumination beam off the second surface of the pellicle.
 4. The methodof claim 2 wherein the second optical metrology model of the pelliclecovering the photomask takes into account the optical effects on thedetection beam related to the reflection off the second surface of thepellicle, refractive indices of the pellicle, and reflection of thedetection beam off the first surface of the pellicle.
 5. The method ofclaim 1 wherein the first and second optical metrology models of thepellicle and the optical metrology model of the photomask includestaking into account reduction of the illumination beam or detection beamintensities due to reflection at the surfaces of the pellicle.
 6. Themethod of claim 1 where the pellicle comprises a material that istransparent to the lithography wavelength.
 7. The method of claim 1wherein the pellicle comprises a polymer film or a glass plate.
 8. Themethod of claim 1 the effect of the material of the pellicle on theillumination and detection beams is expressed as a function of therefractive indices.
 9. The method of claim 1 wherein the mediumseparating the pellicle and the photomask is air or a liquid.
 10. Themethod of claim 1 wherein the one or more profile parameters of the maskstructure include critical dimensions of the photomask structure. 11.The method of claim 1 further comprising: comparing the one or moreprofile parameters to acceptable ranges of values for the profileparameter.
 12. The method of claim 11 wherein photomasks with profileparameters outside of the acceptable ranges of values for the one ormore profile parameters are flagged.
 13. The method of claim 1 whereinthe angle of incidence of the illumination beam is selected from thegroup consisting of zero and greater than zero degrees.
 14. The methodof claim 1 wherein determining one or more profile parameters of themask structure using the optimized integrated optical metrology model isdone: (a) using regression or (b) using a generated library of photomaskstructure profile parameters and pellicle parameters and correspondingsimulated diffraction signals or (c) using a machine learning systemtrained to receive as input a measured diffraction signal and generatean output of corresponding photomask structure profile parameters andpellicle parameters.
 15. The method of claim 1 wherein the pelliclecomprises more than one layer of polymer film or more than one glassplate.
 16. The method of claim 15 wherein the first optical model of thepellicle takes into account optical effects on the illumination beamrelated to the reflection off the first surface of the first layer ofthe pellicle, refractive indices of the first layer of the pellicle, andreflections off the second surface of the first layer of the pellicleand the reflection off the first surface of the subsequent layers of thepellicle, refractive indices of the subsequent layers of the pellicle,and reflections off the second surface of the subsequent layers of thepellicle.
 17. A method of characterizing a photomask covered with apellicle for quality control purposes, the method comprising: developinga first optical metrology model of a pellicle covering a photomask, thepellicle having a first and second surface; developing an opticalmetrology model of the photomask, the photomask separated from thepellicle by a medium and having a structure, the structure havingprofile parameters, the optical metrology model of the photomask takinginto account the optical effects on the illumination beam transmittedthrough the pellicle and diffracted by the photomask structure, thediffraction generating a detection beam; developing a second opticalmetrology model of the pellicle, the second optical metrology model ofthe pellicle taking into account optical effects on the detection beamrelated to the first and second surfaces of the pellicle; integratingthe first and second optical metrology models of the pellicle and theoptical metrology model of the photomask, generating an integratedoptical metrology model; optimizing the integrated optical metrologymodel; determining one or more profile parameters of the mask structureusing the optimized integrated optical metrology model; and comparingthe determined one or more profile parameters of the mask structure toacceptable ranges of the one or more profile parameters.
 18. The methodof claim 17 wherein the first optical metrology model of the pelliclecovering the photomask takes into account the optical effects on theillumination beam related to reflection off the first surface of thepellicle, refractive indices of the pellicle, and reflection of theillumination beam off the second surface of the pellicle.
 19. Acomputer-readable storage medium containing computer-executableinstructions to determine one or more profile parameters of a photomaskcovered with a pellicle, the method comprising instructions for:developing a first optical metrology model of a pellicle covering aphotomask, the pellicle having a first and second surface; developing anoptical metrology model of the photomask, the photomask separated fromthe pellicle by a medium and having a structure, the structure havingprofile parameters, the optical metrology model of the photomask takinginto account the optical effects on the illumination beam transmittedthrough the pellicle and diffracted by the photomask structure, thediffraction generating a detection beam; developing a second opticalmetrology model of the pellicle, the second optical metrology model ofthe pellicle taking into account optical effects on the detection beamrelated to the first and second surfaces of the pellicle; integratingthe first and second optical metrology models of the pellicle and theoptical metrology model of the photomask, generating an integratedoptical metrology model; optimizing the integrated optical metrologymodel; and determining one or more profile parameters of the maskstructure using the optimized integrated optical metrology model.
 20. Asystem for determining one or more profile parameters of a photomaskcovered with a pellicle, the system comprising: a photomask covered by apellicle, the photomask configured for use in a lithography process forfabricating structures in a wafer, the pellicle having a first andsecond surface, the photomask separated from the pellicle by a mediumand having a structure, the structure having profile parameters; anintegrated optical metrology model of the photomask covered by thepellicle comprising: a first optical metrology model of the pellicle,the first optical metrology model of the pellicle taking into accountoptical effects on an illumination beam related to reflection off thefirst surface of the pellicle, refractive indices of the pellicle, andreflection off the second surface of the pellicle; an optical metrologymodel of the photomask taking into account the optical effects on theillumination beam transmitted through the pellicle and diffracted by thephotomask structure, the diffraction generating a detection beam; asecond optical metrology model of the pellicle taking into accountoptical effects on the detection beam related to reflections off thesecond surface of the pellicle, refractive indices of the pellicle, andreflections off the first surface of the pellicle; an optical metrologysystem configured to determine the one or more profile parameters of thephotomask structure, the optical metrology system comprising: a beamsource and detector configured to measure a diffraction signal off thephotomask structure; and a processor configured to compare the measureddiffraction signal to a simulated diffraction signal, wherein thesimulated diffraction signal is associated with one or more values ofone or more profile parameters, wherein the simulated diffraction signalwas generated using one or more values of one or more profile parametersand the integrated optical metrology model.